This invention relates to an AM broadcasting system used to transmit stereo signals in the AM broadcasting band.
In the past, the Harris Corp. presented an example of an AM stereo broadcasting system using two carriers having the same frequency and a 90-degree phase difference from each other. The details of this system are described in "A Linear AM Stereo System Using Quadrature Modulation" by Clifford Leitch and David L. Hershberger (IEEE Transactions of Broadcasting, Vol. BC-24, No. 3, September 1978). In the system proposed by the Harris Corp., a pilot signal for stereo signal identification is represented by a subsonic tone which is added to the (L-R) signal, as is obvious in FIG. 2 on page 61 of the above-mentioned article. The demodulation is carried out by the synchronous detection of the (L-R) signal, as illustrated in FIG. 3 on the same page of the article. In such a method, in order to modulate (L-R) signal sufficiently, the modulation depth of the pilot signals must be minimized; devices including filters must be incorporated in the circuit used for the pilot signal detection; and an insufficient modulation depth for the pilot signal, when the carrier-noise ratio is small, makes it difficult to distinguish the pilot signal from noise.
To solve these problems, this invention proposes an AM stereo broadcasting system which is capable of increasing the modulation depth of pilot signals irrespective of the stereo signals.